Zernike fails to encompass all influences on VA, study finds

Zernike polynomials do not fully describe the surface of the cornea, a study suggests. The failure of this method to model all the features of the corneal shape that influence vision may confound clinical diagnosis and treatment with wavefront-guided correction systems, according to study authors Michael Smolek, PhD, and Stephen Klyce, PhD.

The LSU Eye Center researchers challenged the assumption that wavefront error data arising from aberrometry is adequately described by a Zernike polynomial function. They obtained corneal topography maps from complex eyes — 32 keratoconus cases, 27 postoperative penetrating keratoplasty cases and 29 postoperative conductive keratoplasty cases. Best corrected visual acuity (BCVA) ranged from 20/12.5 to 20/400. Topography was analyzed to determine wavefront error and the elevation fit error for a 4-mm optical zone.

The correlation of BCVA to wavefront error for fourth-order terms was moderately strong and significant (P < .001). Results were similar for tenth-order correlations, but there was no significant difference between the two regressions.

“Only 72% of the cases showed a shift toward increased wavefront error with the 10th-order series, whereas 18% lost wavefront error,” the authors said in the November issue of Investigative Ophthalmology & Visual Science.

“The wavefront error correlation to acuity was moderately strong, but the corneal elevation fit error strongly correlated with visual acuity, indicating that Zernike polynomials do not fully characterize the surface shape features that influence vision,” the authors concluded. Extrapolating to a larger expansion series only led to unpredictable increases and decreases, they said.